Feline Leukemia Virus (FeLV) is a common cause of infection of domestic cats throughout the world and a cause of significant morbidity and mortality. The prevalence of antigenaemia may vary from 1 to 5 percent in healthy cats to 15 to 30 percent in sick cats (Hosie M. J. et al., Veterinary Records, 1989, 128, 293-297; Braley J., Feline Practice, 1994, 22, 25-29; Malik R. et al., Australian Veterinary Journal, 1997, 75, 323-327; Arjona A. et al., Journal of Clinical Microbiology, 2000, 38, 3448-3449). The virus may establish a life-long infection characterized by a persistent viraemia and a fatal outcome. Most FeLV-related diseases occur persistently in infected animals, and they are always serious and most likely fatal. Among the most frequently diagnosed conditions are lymphomas, myeloid leukaemias, immunodeficiency and non-regenerative anaemia. The infection can be controlled by the identification and isolation of persistently viraemic cats, which are the source of the infection. Vaccines have also helped to prevent the virus spreading. Several FeLV vaccines are available. Most of them contain either inactivated virus or recombinant subunits. Their efficacy is controversial (Sparkes A. H., Journal of Small Animal Practice, 1997, 38, 187-194). Vaccine breakdowns have been observed.
An alternative way would be to use recombinant viral vector. The canarypox virus vector and especially the ALVAC vector have been tested for the expression of FeLV genes (Tartaglia J. et al., Journal of Virology, 1993, 67, 2370-2375; Poulet H. et al., Veterinary Record, 2003, 153, 141-145). A commercial recombinant FeLV vaccine is also available (EURIFEL® FeLV, Merial).
The FeLV genome codes for three genes: a GAG gene coding for the major structural components of the virus, an ENV gene which codes for the envelope glycoprotein, and a POL gene cndoing the polymerase protein (Thomsen D. R., et al., Journal of General Virology, 73, 1819-1824, 1992). The FeLV envelope (ENV) gene encodes a gp85 precursor protein which is proteolytically processed by cellular enzymes(s) to yield the major envelope glycoprotein gp70 and the associated transmembrane protein p15E (DeNoronha, F., et al., 1978, Virology 85:617-621; Nunberg, J. H., et al., 1983, PNAS 81:3675-3679). The transmembrane protein p15E contains a sequence conserved among gammaretroviruses with immunosuppressive properties (Mathes, L. E. et al., 1978, Nature). FeLV envelope glycoprotein is one of the major immunogens and is the target of FeLV-specific cytotoxic T cell responses as well as neutralizing antibodies (Flynn, J. N., et al., 2002, J. Virol.). US patent application US 2008/0008683 discussed a polypeptide that is capable of modulating the immunosuppressive properties of a viral protein against the host in which it is expressed. The FeLV GAG gene encodes a precursor polyprotein which is cleaved by the protease (FeLV PRO gene) to generate the capsid proteins. The capsid proteins are also a major immunogen inducing FeLV-specific cytotoxic T cell responses as well as neutralizing antibodies (Flynn, J. N., et al., 2002, J. Virol.). The POL gene encodes three proteins: protease (PRO), reverse transcriptase and integrase. Autoprocessing by the protease portion of the gene gives rise to all three proteins of the POL region (Thomsen D. R., et al., 1992).
There is a general need for an improvement in efficacy and safety of the FeLV vaccines and for more effective protection in field conditions.